Amplifier

ABSTRACT

A re-circulating amplifier comprising a SAW filter with group delay characteristics for providing a time delay function.

[0001] This invention relates to the field of amplifiers and moreparticularly, but not exclusively to such amplifiers used in radiofrequency receivers and transceivers.

[0002] There is an ever increasing requirement in the field ofcommercial radio based products for reductions in the size and energyrequirements of radio frequency (RF) receivers and transceivers. Suchreceivers and transceivers are required to exhibit very high reliabilityfor use in systems where products may be required to be deployed for along periods of time powered by only a small power source.

[0003] Additionally such receivers and transceivers are typicallyrequired to exhibit good sensitivity, selectivity and freedom fromspurious responses. Exceptionally such devices must minimise oreliminate any electrical emissions (such as provided by a localoscillator or the instability in an amplifier).

[0004] Low current small size receivers capable of partially meeting therequirements above include designs based on a ‘crystal’ detector, TRF,super-regen and sequential amplifiers.

[0005] Each of these components has inherent problems, including theinadequate sensitivity of the ‘crystal’ the TRF (Tuned Radio Frequency)being limited in its sensitivity by the RF gain and the isolationbetween the antenna and detector. Additionally the selectivity of TRF isnot particularly good.

[0006] Furthermore the super-regen is inherently unstable and is liableto emit radiation on the receive frequency.

[0007] A receiver based on the sequential amplifier goes furthest towardmeeting any new exacting requirements. This device uses a surfaceacoustic wave (SAW) band-pass RF filter for frequency determination withtwo (or more) amplifiers linked through a SAW delay line. Each amplifierstage is energised alternately so that inadvertent coupling between theamplifiers does not produce instability. However, this topology islimited by several factors:

[0008] a) The state of the art configurations use two SAW devices, afilter to define the operational frequency and a delay line to providethe necessary time delay.

[0009] b) Ideal SAW delay lines suffer an intrinsic 6 dB insertion loss,real delay lines suffer additional losses caused by many factorsincluding the efficiency of transducers and propagation along thedevice. Loss in the delay line must be compensated by higher gain in theamplifiers that consume greater supply current.

[0010] c) Conventional SAW delay lines must be sufficiently long toaccommodate the transmission path that provides the requisite delay.Methods of folding the transmission path are available (to make thedevice smaller) at the expense of insertion loss. Thus devices withappreciable delay are physically large and may be the limiting factor indefining the dimensions of the receiver.

[0011] d) The amplifiers are energised with a high duty cycle (i.e. 50%)

[0012] Accordingly there is provided a re-circulating amplifiercomprising a SAW filter with group delay characteristics saidcharacteristics providing a time delay function.

[0013] In a preferred embodiment of the invention a re-circulatingamplifier is provided wherein the SAW filter comprises a combination ofcharacteristics including group delay and frequency selectivitysimultaneously as part of a sequential amplifier.

[0014] In a further embodiment of the invention a receiver is provided,comprising a re-circulating amplifier, wherein said re-circulatingamplifier does not comprise a local oscillator or super-regenerationmeans.

[0015] In a yet further embodiment of the invention a transceiver isprovided comprising a re-circulating amplifier, wherein saidre-circulating amplifier does not comprise a local oscillator orsuper-regeneration means.

[0016] The invention will now be described by way of example only withreference to th accompanying drawings in which;

[0017]FIG. 1—shows a block diagram of an receiver in accordance with theinvention

[0018]FIG. 2—shows two diagrammatic representations of the attenuationand delay characteristics of an amplifier and filter

[0019]FIG. 3—shows a graph of the supply current consumed by a receiverplotted against time for a range of input signal amplitudes.

[0020]FIG. 4—shows a block diagram of a transceiver in accordance withthe invention

[0021] In the receiver embodiment shown in FIG. 1, an input amplifier 2samples the signal from the antenna 4, amplifies and applies it to theinput to a SAW delay 6 (implemented as the group delay of a filter).When the signal begins to emerge from the SAW delay 6 the inputamplifier 2 is switched off and the second amplifier 8 switched on. Thesignal is amplified and re-applied to the SAW input 6. When the signalemerges for a second time it is again amplified by the second amplifier8. When the signal emerges for a third time, the second amplifier 8 isswitched off. Because only one SAW device 6 is used in the embodimentshown, there is no possibility of frequency misalignment between ‘stags’. Once the signal has completed the amplification process the outputsignal may typically be routed via a low pass filter 10.

[0022] Thus the input amplifier 2 performs a receive sampling action onthe antenna input 4. This sampling action of the amplifier 2 causes asmall loss of potential amplification. For example the duty ratio of theamplifiers 2, 8 will be about 10% and this results in a 10 dB loss inultimate gain, of the circuit. However it is offset by the action of thesecond amplifier 8 which is used to provide about three lots of gain tothe signal. Thus the gain of the circuit approximates to the sum of theinput amplifier 2 plus three times the gain of the second amplifier 8minus the 3 times the attenuation of the SAW device 6 and minus thesampling loss. (In a research prototype this evaluated to approximately25+3*25−3*5−10=75 dB).

[0023] The detection action is performed by monitoring the collectorcurrents of the amplifiers 2, 8, this current being proportional to theinput power to the receiver and performs an AM detection of the commandtones.

[0024] The receiver is devised to use the minimum of electricalcomponents, which aids the reduction of current consumption and physicalsize and provides for improvements in reliability. The use of thisinnovative design provides for circuit elements to be arranged toperform multiple functions.

[0025] A single SAW filter 6 is used to provide a means of determiningthe receiver's centre frequency and the filter's group delaycharacteristics are used to provide an electrical delay as part ofsequential amplifier.

[0026] Two amplifier sections are employed 2, 8, the gating of theamplifier control acts as an RF switch which isolates the stage from itsinput and output. The second stage amplifier 8 re-amplifies the signalseveral times resulting in high net gain. (This re-circulation iscounter to intuition as normally this would result in instability.)

[0027] The definition of the SAW filter 6 characteristics of the groupdelay and the amplitude response control the operation of there-circulating amplifier. These characteristics may be satisfied by a4^(th) order filter design.

[0028] AM detection is performed by a similar technique to that used bysuccessive logarithmic amplifiers, where the supply current rises as aproportion of the input signal level.

[0029] Certain specific difficulties overcome during development of theinvention include,

[0030] Dwell Time

[0031] Sufficient time must be allowed for the signal to die away. Thesurface acoustic wave is reflected at each end of the filter, andalthough there is loss both on transit and reflection, energy is stillpresent after several transits of the filter. If this energy is abovenoise level when the input amplifier is turned on, the receiver will bedesensitised. In practice, when the input amplifier is turned on for 4microseconds, the re-circulating amplifier is on for 16 microseconds, afurther 44 microseconds must be allowed before the input amplifier maybe re-engaged. While this has a benefit of saving supply current it alsodesensitises the receiver.

[0032] Input Sampling

[0033] Input amplifier sampling is a critical aspect of receiverperformance. The input amplifier must be on for at least as long as thetransit delay of the filter, and a certain amount longer than this sothat gating of the input signal occurs after the filter and not before.Any switching of the signal constitutes mixing, and will broaden thepass-band considerably. If the switching occurs before the filter, thefilter shape will be compromised. However, if the amplifier was switchedon before the input is required, no switching edge is seen by the systemat the expense of greater energy consumption.

[0034] At the trailing edge of the input pulse more precision isrequired. There must be no overlap of the switching waveforms otherwiseboth amplifiers will be on together, and instability is likely.Conversely if the gap is too large, signal, and consequentiallysensitivity, is lost as energy dies away in the SAW filter before it canbe amplified by the second stage.

[0035] Noise Amplification

[0036] There is no point in leaving the re-circulating amplifierenergised for a longer period in an attempt to increase the sensitivity.With very small signals the input stage produces noise and once there-circulating amplifier has reached limiting on this input noise thereis no more information about the signal to be gained. In fact leavingthe re-circulating amplifier on too long merely increases currentconsumption and risks instability.

[0037] Max AM Frequency

[0038] With a switching rate of 16 kHz (derived from the 64 μs cycletime) it is theoretically impossible to receive amplitude modulation ata frequency above 8 kHz, however in our prototype the highest frequencyis less than 1 kHz so bandwidth is not a problem. The 16 kHz switchingrate does need to be filtered out, however, and a ratio of 16 to 1 meansthat a 3 pole filter network is necessary. This is provided by theactive filter surrounding the first operational amplifier.

[0039] Off-Frequency Stability

[0040] The second amplifier has its output connected to its input viathe SAW filter. There is a threat that at frequencies where the group dlay is small, a signal could pass quickly from the output back to theinput Because the amplifier is switch d on for a finite time theoff-centre frequency signals could have many more than 4 sets ofamplification. It is important at these frequencies that the attenuationof the filter is greater than the amplifier gain in order to maintainstability.

[0041] With reference to FIG. 2, in the left hand example 12 theattenuation is less than the amplifier gain at the point where there isonly a short delay, which means that it may be unstable. In the righthand example 14 the attenuation is larger than the gain and the circuitis stable.

[0042] Specific Characteristics of the Delay Include;

[0043] Rise Time Control

[0044] The rate at which the RF amplifiers are switched on and off isimportant. In particular switching the amplifiers on too quickly seemsto introduce a noise impulse into the SAW filter. By controlling therise time to be more than 0.5 μs the noise injection is minimised andsensitivity is not degraded.

[0045] In practice control lines are likely to be slew rate limited dueto the impedance of the low current ASIC output stages.

[0046] Successive detection of the signal takes place in the amplifyingtransistors without the need of a separate detector stage (the techniquethat is used in logarithmic amplifiers). FIG. 3 shows the supply currentconsumed by the receiver plotted against time (each graduation being 4μs) for various input signal amplitudes.

[0047] An AM signal comprises a sequence of 100% amplitude modulatedtones. For example if a signal level of −90 dBm were being receivedduring the 100% phase of the modulation then no signal would be presentduring the 0% phase. The energy consumed by the receiver, denoted by thearea under each curve (current×time), is different for the two signallevels and this appears as an alternating voltage at the tone (data)rate

[0048] The collector current of the two amplifier transistors is summed,low pass filtered to remove the switching component, and amplified untillimiting occurs. This output is then ready for digital processing by thedigital ASIC.

[0049] Note: this form of AM detection applies to 100% modulated data ortone sequences and not to analogue AM signals (i.e. those that do nothave modulation levels in the region of 50%).

[0050] In a further embodiment of the invention, a transceiver isprovided. The characteristics of a transceiver produced in accordancewith the invention include the provision of a very small low currentdevice with no local oscillator or active RF source and an inherentranging capability.

[0051] In FIG. 4 an input amplifier 14 samples the signal from anantenna 18, amplifies and applies it to the input to a SAW delay 20.When the signal begins to emerge from the SAW delay 20 the inputamplifier 16 is switched off and the second amplifier 22 is switched on.The signal is amplified and re-applied to the SAW input 20. When thesignal emerges for a second time it is again amplified by the secondamplifier 22. When it emerges for a third time the second amplifier 22is switched off and the transmit amplifier 24 is switched on. The devicenow switches off for a short period while the energy in the SAW delay 20dies away before repeating the cycle.

[0052] Once the recieved signal has completed the amplification process,an output of the signal may typically be routed via a low pass filter26.

[0053] Thus the input amplifier 16 performs a receive sampling action onthe signal at the antenna input 18. The second amplifier 22 is used toprovide substantial gain and the transmit amplifier 24 provides RFenergy back to the antenna 18. Because the return signal is an amplifiedversion of that received, the frequency is identical and the stationlistening to the return signal can use very narrow filters and achievegreat sensitivity.

[0054] There is an implicit (AM) receiver function within the circuitrywhere detection is performed by monitoring the collector currents of theamplifiers, this current is proportional to the input signal level tothe device.

[0055] Ranging can be gauged by measuring the round trip delay from anilluminating station through the device and return. The timings throughthe device are crystal accurate and comprise:

[0056] a) receiver input amplifier 16 which is gated on for fixed period(this was 4 μs in the prototype),

[0057] b) delay through device is not dependant on propagation delaythrough the SAW 20 but controlled by the duration of amplifier 22,

[0058] c) transmitter stage is gated on for fixed period (again 4 μs).

[0059] Long distance ranging can be provided by making the transmitteramplifier 24 transmit for a relatively long period (the 4 μs). Shortdistance ranging can be provided by making the transmitter amplifier 24transmit for a very short period (20 ns). The lower average power of theshort range system can be offset by the reduced propagation loss andhence the signal to noise ratio is maintained.

[0060] Inherent receiver is used to listen to commands addressed to aparticular device. These commands could control equipment to the performspecific actions:

[0061] a) to acquire the timing of the base station illumination,

[0062] b) to provide a return transmission containing data,

[0063] c) to provide long distance ranging transmissions (using widepulses),

[0064] d) to provide short distance relative ranging transmissions(using a high frequency code).

[0065] Data transmission can be achieved using a low data rate OOKmodulation imposed on the return transmission.

[0066] The device will be robust and reliable. The componentssusceptible to mechanical shock are the clock crystal and the SAW delay,both of which exhibit much higher shock resistance than RF crystals. Thedevice has a very low parts count and has high integration of digitalfunctions within the ASIC and this is likely to be very reliable.

[0067] Low probability of intercept is achieved by having no RF sourceswithin the device (there is no LO in the receiver or active source forthe transmitter). Thus a deployed device operating without illuminationwould radiate no signals and the only effect would be a small increasein the noise floor when in intimate proximity with the device. Whenilluminated the incoming signal is likely to dominate any emission fromthe device which is on exactly the same frequency and so the presence ofthe device is difficult to ascertain. When the device is transmittingthe return signal, being on the same frequency as the illumination, isnot particularly noticeable as a new signal and can only be gauged by anapparent increase in the illumination in the vicinity of the device.

[0068] Further the illumination could be hopped over a small frequencyband in a pseudo-random fashion and so the signals would be dispersedand less detectable with a narrow band scanner.

1. A re-circulating amplifier comprising a SAW filter with group delaycharacteristics said characteristics providing a time delay function. 2.A re-circulating amplifier in accordance with claim 2, wherein said SAWfilter comprises a combination of characteristics including group delayand frequency selectivity simultaneously as part of a sequentialamplifier.
 3. A receiver comprising a re-circulating amplifier inaccordance with claim 1 or claim 2, wherein said re-circulatingamplifier does not comprise a local oscillator or super-regenerationmeans.
 4. A transceiver comprising a re-circulating amplifier inaccordance with claim 1 or claim 2 wherein said recirculating amplifierdoes not comprise a local oscillator or super-regeneration means.